This invention relates generally to structural support members and more particularly to prestressed and cambered structural support members.
It is well-known that the load capacity of structural supports such as beams can be increased by prestressing. The prestress is imparted to the beam in a direction opposite to the direction of the stress resulting from the eventual load. When so prestressed, a compression stress is induced at the lower surface of the beam and a tension stress induced at the upper surface of the beam. Both of the stresses are along a line parallel to the length of the beam. The prestress in the beam will be taken up in the loaded condition before the load acting on the beam causes a tension stress to be imparted at the beam lower surface. Accordingly, a prestressed beam is able to carry a greater load than a non-prestressed beam of the same cross section.
This concept has been employed in the making of compound wooden bonded structures commonly used in building construction. One example, is in the form of horizontally laminated board structures commonly referred to as "glue lam" beams. Such beams are comprised of a plurality of one and one-half to two inch thick laminates which are glued together. In making such a structure, adhesive is applied to the laminates which are then bent in a direction opposite to the directon of the future load. When the adhesive cures, the bending forces are removed and the bend, or camber, is maintained by the cured adhesive.
Such a beam can carry an additional load over a non-cambered beam of the same cross section. The additional load is represented by the load required to overcome the bending stress in compression at the bottom surface of the cambered beam. However, the amount of this compression stress is slight, as very little force is required to bend the laminates to the desired camber. Therefore, the compression stress at the bottom of the beam retained by the cured adhesive is very small compared to a design load stress of a noncambered beam of the same cross section. Accordingly, such a beam has only a minimal degree of prestress for stopping sagging in later use.
Of course, the degree of prestressing can be increased by increasing the camber. However, there are practical limits to the degree of cambering which are dictated by the required installed configuration of the beam. The amount of camber is also limited by the degree to which the laminates can be bent before failing. Heretofore, to support significant loads over great distances with horizontally laminated beams having industry acceptable degrees of camber, the beam size is simply increased until the desired load can be carried.
Increasing the prestress in beams has also been attempted in ways such as shown in U.S. Pat. No. 3,294,608 to Peterson. Peterson discloses a cambered beam having a wooden beam which overlies a comparatively thin tension element made of metal. In making such a beam, the adhesive is applied between the members and then the tension element is longitudinally stressed. After the adhesive cures, the longitudinal tension forces are removed thus prestressing the beam. The Peterson patent alleges that such a structure may be capable of carrying a load one hundred percent greater than that of a non-prestressed beam. However, such a structure is difficult and expensive to manufacture and therefore not believed to be practical.
In another process used in making compound bonded wooden structures, the cross section of the beam is made from three separate parts. The middle part is thicker than the outer parts and pre-bent by steam in a direction opposite to the eventual load. The outer two parts are bent in the opposite direction, the direction of the eventual load. The members are then straightened and bonded together resulting in a straight, prestressed beam. For example, U.S. Pat. No. 2,039,398 to Dye discloses such a beam wherein the top and bottom members are tension and compression prestressed respectively by applying appropriate longitudinal forces. However, the Dye structure is shown as either being straight or having a camber in the direction of the load. Additionally, steaming beams to cause them to bend and thereafter gluing them together is not practical. It would be very difficult to control the steaming process to produce repeatable curvature of the members for a given time period of steaming, etc.
Another structure prestressed by imparting longitudinal forces to interconnected beam members is disclosed in U.S. Pat. No. 4,500,378 to Reppel et al. However, it is very difficult to longitudinally impart significant stresses in the individual beams. The Reppel et al. process and apparatus for so doing are very complex.